Needed to build secp-sys 0.5 and secp-sys 0.6 in the same tree. Fixes #489. This PR can be reproduced by running ./vendor-libsecp.sh depend/ 0_6_1 a1102b12196ea27f44d6201de4d25926a2ae9640 in the secp256k1-sys directory.
Two API changes needed to be reflected: schnorrsig_sign and schnorrsig_verify. Also bump both Cargo.toml files